FDM-assisted OA-CEAS system for simultaneous measurements of temperature, CO2, and CO in flames

A system based on off-axis cavity enhanced absorption spectroscopy (OA-CEAS) integrated with frequency division multiplexing (FDM)-assisted wavelength modulation spectroscopy (WMS) was developed for simultaneous measurements of temperature, CO₂, and CO in flames by employing two distributed feedback (DFB) diode lasers at 2.0 µm and 2.3 µm. An integrated cavity with an open-path configuration was made up of two cavity mirrors with a reflectivity of 99.8 %, and an effective optical absorption path length of approximately 5.2 m was achieved within a 6 cm diameter flame region. A CO₂ line pair at 4990.418 cm⁻¹ and 4990.664 cm⁻¹, along with a CO line at 4300.699 cm⁻¹, were selected as the measurement targets. The system was initially calibrated in a heated static cell and subsequently tested in C₂H₄/air premixed flat flames for six different equivalence ratios. All concentrations measured by the 1f-normalized 2f signals were compared with the results obtained from both the direct absorption signals and simulations based on adiabatic chemical equilibrium calculations using Chemkin. Additionally, the measured temperatures were compared with the thermocouple readings. According to the measurements conducted at representative C₂H₄ flames with different equivalence ratios, the accuracies for the measurements of temperature, CO₂ concentrations, and CO concentrations were ∼1.062 %, ∼1.082 %, and ∼1.052 %, respectively. All the measurements illustrate the potential of the system for combustion diagnosis.